silane

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Silane is an
inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistr ...
with
chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
, . It is a colourless,
pyrophoric A substance is pyrophoric (from grc-gre, πυροφόρος, , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolith ...
, toxic gas with a sharp, repulsive smell, somewhat similar to that of
acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bond ...
. Silane is of practical interest as a precursor to elemental
silicon Silicon is a chemical element with the Symbol (chemistry), symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a Tetravalence, tetravalent metalloid and semiconductor. It is a member ...
. Silane with
alkyl group In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloalk ...
s are effective water repellents for mineral surfaces such as concrete and masonry. Silanes with both organic and
inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistr ...
attachments are used as coupling agents.

# Production

## Commercial-scale routes

Silane can be produced by several routes. Typically, it arises from the reaction of hydrogen chloride with magnesium silicide: : Mg2Si + 4 HCl -> 2 MgCl2 + SiH4 It is also prepared from metallurgical-grade silicon in a two-step process. First, silicon is treated with
hydrogen chloride The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chlorid ...
at about 300 °C to produce trichlorosilane, HSiCl3, along with
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the chemical ...
gas, according to the
chemical equation A chemical equation is the symbolic representation of a chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. ...
: Si + 3 HCl -> HSiCl3 + H2 The trichlorosilane is then converted to a mixture of silane and
silicon tetrachloride Silicon tetrachloride or tetrachlorosilane is the inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorga ...
: : 4 HSiCl3 -> SiH4 + 3 SiCl4 This redistribution reaction requires a catalyst. The most commonly used catalysts for this process are
metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...
halides, particularly
aluminium chloride Aluminium chloride, also known as aluminium trichloride, is an inorganic compound with the formula . It forms hexahydrate with the formula , containing six water molecules of hydration. Both are colourless crystals, but samples are often conta ...
. This is referred to as a redistribution reaction, which is a double displacement involving the same central element. It may also be thought of as a
disproportionation In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term ca ...
reaction, even though there is no change in the oxidation number for silicon (Si has a nominal oxidation number IV in all three species). However, the utility of the oxidation number concept for a covalent molecule, even a polar covalent molecule, is ambiguous. The silicon atom could be rationalized as having the highest formal oxidation state and partial positive charge in and the lowest formal oxidation state in , since Cl is far more electronegative than is H. An alternative industrial process for the preparation of very high-purity silane, suitable for use in the production of semiconductor-grade silicon, starts with metallurgical-grade silicon, hydrogen, and
silicon tetrachloride Silicon tetrachloride or tetrachlorosilane is the inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorga ...
and involves a complex series of redistribution reactions (producing byproducts that are recycled in the process) and distillations. The reactions are summarized below: : Si + 2 H2 + 3 SiCl4 -> 4 SiHCl3 : 2 SiHCl3 -> SiH2Cl2 + SiCl4 : 2 SiH2Cl2 -> SiHCl3 + SiH3Cl : 2 SiH3Cl -> SiH4 + SiH2Cl2 The silane produced by this route can be thermally decomposed to produce high-purity silicon and hydrogen in a single pass. Still other industrial routes to silane involve reduction of silicon tetrafluoride () with
sodium hydride Sodium hydride is the chemical compound with the empirical formula NaH. This alkali metal hydride is primarily used as a strong yet combustible base (chemistry), base in organic synthesis. NaH is a saline (salt-like) hydride, composed of Na+ and ...
(NaH) or reduction of with lithium aluminum hydride (). Another commercial production of silane involves reduction of
silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical comp ...
() under Al and gas in a mixture of
NaCl Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and ...
and aluminum chloride () at high pressures: : 3 SiO2 + 6 H2 + 4 Al -> 3 SiH4 + 2 Al2O3

## Laboratory-scale routes

In 1857, the German chemists Heinrich Buff and Friedrich Woehler discovered silane among the products formed by the action of
hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is a component of the gas ...
on aluminum silicide, which they had previously prepared. They called the compound ''siliciuretted hydrogen''. For classroom demonstrations, silane can be produced by heating
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a soil texture, textur ...
with
magnesium Magnesium is a chemical element with the Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group ...
powder to produce magnesium silicide (), then pouring the mixture into hydrochloric acid. The magnesium silicide reacts with the acid to produce silane gas, which burns on contact with air and produces tiny explosions.. This may be classified as a
heterogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the Uniformity (chemistry), uniformity of a Chemical substance, substance or organism. A material or image that is homogeneous is uniform in compos ...
acid–base chemical reaction, since the isolated ion in the
antifluorite
structure can serve as a Brønsted–Lowry base capable of accepting four protons. It can be written as : 4 HCl + Mg2Si -> SiH4 + 2 MgCl2 In general, the alkaline-earth metals form silicides with the following stoichiometries: , , and . In all cases, these substances react with Brønsted–Lowry acids to produce some type of hydride of silicon that is dependent on the Si anion connectivity in the silicide. The possible products include and/or higher molecules in the homologous series , a polymeric silicon hydride, or a
silicic acid Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
. Hence, with their zigzag chains of anions (containing two lone pairs of electrons on each Si anion that can accept protons) yield the polymeric hydride . Yet another small-scale route for the production of silane is from the action of
sodium amalgam Sodium amalgam, commonly denoted Na(Hg), is an alloy of mercury and sodium. The term amalgam is used for alloys, intermetallic compounds, and solutions (both solid solutions and liquid solutions) involving mercury as a major component. Sodium ...
on dichlorosilane, , to yield monosilane along with some yellow polymerized silicon hydride .

# Properties

Silane is the
silicon Silicon is a chemical element with the Symbol (chemistry), symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a Tetravalence, tetravalent metalloid and semiconductor. It is a member ...
analogue of
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
. Because of the greater electronegativity of hydrogen in comparison to silicon, this Si–H bond polarity is the opposite of that in the C–H bonds of methane. One consequence of this reversed polarity is the greater tendency of silane to form complexes with transition metals. A second consequence is that silane is
pyrophoric A substance is pyrophoric (from grc-gre, πυροφόρος, , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolith ...
— it undergoes spontaneous combustion in air, without the need for external ignition. However, the difficulties in explaining the available (often contradictory) combustion data are ascribed to the fact that silane itself is stable and that the natural formation of larger silanes during production, as well as the sensitivity of combustion to impurities such as moisture and to the catalytic effects of container surfaces causes its pyrophoricity. Above 420 °C, silane decomposes into silicon and
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the chemical ...
; it can therefore be used in the
chemical vapor deposition Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (electro ...
of silicon. The Si–H bond strength is around 384 kJ/mol, which is about 20% weaker than the H–H bond in H2. Consequently, compounds containing Si–H bonds are much more reactive than is H2. The strength of the Si–H bond is modestly affected by other substituents: the Si–H bond strengths are: SiHF3 419 kJ/mol, SiHCl3 382 kJ/mol, and SiHMe3 398 kJ/mol.

# Applications

While diverse applications exist for organosilanes, silane itself has one dominant application, as a precursor to elemental silicon, particularly in the semiconductor industry. The higher silanes, such as di- and trisilane, are only of academic interest. About 300
metric ton The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a International System of Units#Non-SI units accepted for use with SI, non-SI unit accepted for use with SI. It is also referred to as a metric ton to disting ...
s per year of silane were consumed in the late 1990s. Low-cost solar photovoltaic module manufacturing has led to substantial consumption of silane for depositing (PECVD) hydrogenated
amorphous silicon Amorphous silicon (a-Si) is the non-crystalline form of silicon used for solar cells and thin-film transistors in liquid-crystal display, LCDs. Used as semiconductor material for a-Si solar cells, or thin-film silicon solar cells, it is deposi ...
(a-Si:H) on glass and other substrates like metal and plastic. The PECVD process is relatively inefficient at materials utilization with approximately 85% of the silane being wasted. To reduce that waste and the
ecological footprint The ecological footprint is a method promoted by the Global Footprint Network to measure human demand on natural capital, i.e. the quantity of nature it takes to support people or an economy. It tracks this demand through an ecological accoun ...
of a-Si:H-based solar cells further several recycling efforts have been developed.

# Safety and precautions

A number of fatal industrial accidents produced by combustion and detonation of leaked silane in air have been reported. Due to weak bonds and hydrogen silane is a pyrophoric gas (capable of autoignition at temperatures below ). :SiH4 + 2 O2 -> SiO2 + 2 H2O$\Delta H = -1517 \text = -47.23 \text$ :SiH4 + O2 -> SiO2 + 2 H2 :SiH4 + O2 -> SiH2O + H2O :2 SiH4 + O2 -> 2 SiH2O + 2H2 :SiH2O + O2 -> SiO2 + H2O For lean mixtures a two-stage reaction process has been proposed, which consists of a silane consumption process and a hydrogen oxidation process. The heat of condensation increases the burning velocity due to thermal feedback. Diluted silane mixtures with inert gases such as
nitrogen Nitrogen is the chemical element with the Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the pnictogens. It is a common element in the ...
or
argon Argon is a chemical element with the Symbol (chemistry), symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 Parts-per notatio ...
are even more likely to ignite when leaked into open air, compared to pure silane: even a 1% mixture of silane in pure nitrogen easily ignites when exposed to air. In Japan, in order to reduce the danger of silane for amorphous silicon solar cell manufacturing, several companies began to dilute silane with
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the chemical ...
gas. This resulted in a symbiotic benefit of making more stable solar photovoltaic cells as it reduced the Staebler–Wronski effect. Unlike methane, silane is fairly toxic: the lethal concentration in air for rats ( LC50) is 0.96% (9,600 ppm) over a 4-hour exposure. In addition, contact with eyes may form
silicic acid Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
with resultant irritation. In regards to occupational exposure of silane to workers, the US
National Institute for Occupational Safety and Health The National Institute for Occupational Safety and Health (NIOSH, ) is the List of United States federal agencies, United States federal agency responsible for conducting research and making recommendations for the prevention of work-related occ ...
has set a recommended exposure limit of 5 ppm (7 mg/m3) over an eight-hour time-weighted average.

* Binary silicon-hydrogen compounds (sometimes called silanes) * Silanization * Magnesium silicide

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